During the drilling of underground wells it is common to utilize downhole motors, particularly if the wellbore needs to be directionally drilled. Downhole motors are very well known, an example of the prior art can be found in U.S. Pat. No. 6,561,290.
Albert Bodine is the patentee of a number of patents related to the technology of downhole cycloidal drill bits (U.S. Pat. No. 4,266,619), mechanically nutating drills (U.S. Pat. No. 4,261,425) and elastically vibrating drills (U.S. Pat. No. 4,271,915). None of these patents contemplate rotation of the drill bit with a drilling motor simultaneously with nutation of the drill bit.
The application of vibratory forces such as oscillations to a pipe string in a wellbore may be used to reduce frictional forces that impede the progression of the pipe string through the wellbore. Various types of vibratory forces have been contemplated for this purpose. For example, the vibratory forces may be longitudinal, transverse or torsional in nature (or perhaps a combination of different forces). Non-limiting examples of devices which generate transverse vibratory forces to reduce frictional forces are described in U.S. Patent Application Publication No. 2012/0160476 (Bakken) and/or PCT International Publication No. WO 2012/083413 A1 (Bakken).
In U.S. Pat. No. 6,279,670 (Eddison et al), drive means such as a positive displacement motor are used to rotate a first member of a valve relative to a second member of a valve in order to vary the flow rate of fluid through a pressure responsive device such as a shock tool, thereby varying a vibration frequency of the pressure responsive device, on the basis that the vibration frequency is generally proportional to the flow rate.
In both U.S. Pat. No. 6,009,948 (Flanders et al) and U.S. Patent Application Publication No. 2012/0048619 (Seutter et al), the vibration frequency of a “resonance tool” and a “drilling agitator tool” respectively are adjusted to achieve a resonant frequency of the system, based upon feedback from downhole sensors which measure the tool responses downhole. In both cases, the vibration frequency is adjusted incrementally until an acceptable excitation level of the pipe string is obtained.
In U.S. Pat. No. 7,730,970 (Fincher et al), controlled oscillations are superimposed on steady drill bit rotation in order to maintain a selected rock fracture level as stress energy stored in an earthen formation is released when fracture of the rock is initiated. In some embodiments of Fincher et al, a control unit performs a frequency sweep to determine an oscillation that optimizes the cutting action of the drill bit and configures the oscillation apparatus accordingly.
There are disadvantages to all of the above approaches. Eddison et al does not allow for changes to be made to the vibration frequency of the pressure responsive device without changing the fluid flow rate through the pipe string. Flanders et al, Seutter et al and Fincher et al all rely on potentially complex sensors and electronic control systems which may be prone to failure in the wellbore environment.